How big is a 1 karat diamond?

It depends on the shape. For e.g. a 1 carat round brilliant diamond has a width of 6,5 mm. Other shapes, such as a marquise diamond is cut to be a longer shape. A one carat marquise diamond would be about 10 mm x 4,75 mm. Oval, pear and emerald along with the marquise shape visually appear larger than the round and square shapes.